1. Field of the Invention
The invention relates to air cooled heat exchangers, such as combustion/gas turbine engine intercoolers or engine rotor coolers, as well as combustion/gas turbine engine or steam turbine engine condensers, which employ external liquid coolant spray or misting of the heat exchanger conduits. More particularly the invention relates to air cooled heat exchangers that employ pressurized gas-entrained liquid coolant misting devices, such as pressurized gas/pressurized liquid coolant emitters or pressurized gas/non-pressurized liquid coolant ejectors/jet pumps, to enhance the heat exchanger's cooling efficiency. Such misting devices generate misting droplets with low likelihood of system clogging attributable to debris in the cooling liquid.
2. Description of the Prior Art
In a steam turbine power generation cycle spent steam exiting the turbine is cooled, condensed back to “clean” or “treated” working liquid and recycled through a boiler. The regenerated steam exiting the boiler is used to power the steam turbine, repeating the cycle. In a combined cycle combustion/steam turbine power plant a heat recovery steam generator (HRSG) that extracts heat from the combustion turbine exhaust is utilized to reheat condensed working liquid, rather than a separate boiler. Water cooled heat exchangers are commonly used to cool and condense spent steam that exits the steam turbines. The cooling water source is often a large body of water, such as a lake, river or the ocean. In some power plant locations it is not feasible to use a large body of water as a cooling source for water cooled heat exchangers.
An air cooled condenser (ACC) for cooling and condensing steam flow from turbines is often substituted for a water cooled heat exchanger where a large body of cooling water is not available or when federal, state or local regulations require a power plant operator to reduce cooling water consumption. One or more fans in the ACC enclosure or structure direct cooling air flow over the heat exchanger conduits that transport the working fluid condensing steam. However, unlike a water cooled heat exchanger, ACC heat transfer performance is dependent on heat capacitance of varying ambient air temperature at the power plant, as compared to the relatively predictable and unlimited heat capacitance and transfer rate of a large body of natural water. If the ambient temperature is extremely hot, i.e. a 41° C./105° F. day the overall performance of the steam turbine reduces dramatically due to insufficient cooling capacity available from an ACC. The lack of cooling requires the steam turbine (ST) to run at lower mass flow rates due to higher ST back pressures in order to prevent the steam turbine from entering an alarm state or even tripping, which reduces the overall performance of the power plant during the hot summer months. To address this condition, plant operators sometimes add fogging or misting evaporative systems under the ACC to lower the ambient air temperature and improve the performance of the ACC.
Currently known commercially available evaporative misting systems have been installed under ACCs to reduce the dry bulb temperature of the ambient air closer to the wet bulb temperature. One goal of the evaporative system is to atomize the water into as fine droplets as possible. The smaller the droplet the faster the water can evaporate, and the closer the nozzles can be to the inlet of the cooling fans that circulate the ambient cooling air throughout the ACC enclosure. A typical pressurized water nozzle with a high flow rate above 1 gallon per minute (GPM)/3.8 liters per minute (LPM) requires the average atomized size of the droplet to be on the order of 50-100 microns in diameter and requires water pressures on the order of 1000 PSI. There are fewer current commercially available pressurized nozzle options available, which are capable of producing 10 micron water droplets, but they cannot produce high flow rates at that water size due to the small orifices required. Over time the small nozzle orifices are prone to clogging with debris carried by the cooling water.
Commercially available jet pump or fluid ejector misting devices utilize pressurized gas flow through a nozzle to create a pressure drop and turbulence in an entrainment zone downstream of the nozzle outlet. A liquid feed source is in fluid communication with the entrainment zone. Pressure drop and turbulence in the entrainment zone suctions liquid from the liquid feed source. For example, in a naval pumping configuration, the ejector is used to remove non-pressurized standing water containing debris from ship compartments by suctioning the water, where it is subsequently entrained in the pressurized gas flow. In other applications ejectors can be utilized as misters by incorporating a diffuser downstream the pressurized gas nozzle outlet, so that the entrained fluid is dispersed as a mist.
Commercially available fire suppression misting devices or emitters utilize a non-combustible pressurized gas source, such as nitrogen, that is directed through a convergent nozzle to entrain pressurized water or fire retardant foam into a fogging mist. Entrained water droplets can be formed in a size range below 50 microns and as small as approximately 10 microns. Pressurized liquid sprayed through a nozzle orifice and the pressurized nitrogen sprayed from the converging nozzle outlet is directed toward a downstream deflector that entrains atomized droplets of the liquid into the nitrogen stream. The droplet atomizing deflector allows the pressurized liquid to be dispensed from relatively large diameter nozzle orifices that are not readily clogged.